Method of manufacturing a product from steel and product manufactured according to said method

ABSTRACT

Method of manufacturing a product ( 1, 15 ) from steel, in which a steel with a metastable first phase in which the steel has first properties is manufactured and, by mechanical influences, is transformed into a second, thermodynamically more stable phase with second properties which are at least partly different from the first properties, and, for adapting the product to particular applications, the product ( 1, 15 ) is manufactured locally differently by modification with regard to said phases with different properties. A product according to the invention therefore has, over its surface and/or its thickness, at least two differently adapted areas ( 10, 11 ) with different properties of the steel.

[0001] The invention relates to a method of manufacturing a product from steel, in which a steel with a metastable first phase in which the steel has first properties is manufactured and, by mechanical influences, is transformed into a second, thermodynamically more stable phase with second properties which are at least partly different from the first properties.

[0002] The invention also relates to a steel product which, by virtue of its composition and its manufacture, has a metastable first phase in which the steel has first properties and which, by mechanical influences, can be transformed into a thermodynamically more stable, second phase with second properties which are at least partly different from the first properties.

[0003] Steel grades have become known, the mechanical properties of which are determined by what is known as the TRIP (Transformation Induced Plasticity) effect. The use of such a steel as a material for reinforcing motor car body sheet parts is known from DE 197 27 759 C2. This steel has a composition of 1 to 6% Si, 1 to 8% Al, 10 to 30% Mn, the rest being essentially iron including the usual accompanying elements in steel, where Al+Si≦12% (all quantities are percentages by mass).

[0004] In these steels, the metastable first phase is formed by metastable austenite which is transformed under mechanical stress into α′ and/or ε martensite or into bainite. Accordingly, when deformation of the material takes place in material processing or when component stress takes place, the metastable austenitic phase is transformed into a thermodynamically more stable phase, in particular martensite or bainite. In the case of plastic deformation of the material, this transformation leads to the flow processes at the slip planes being stopped, so that the overall deformation takes place successively at a great number of slip planes, hardening of the material occurring at the same time. In this way, high strengths with at the same time high ductility are achieved, in particular in the case of high uniform elongation, that is elongation without local necking. As the transition from the first phase to the second phase does not take place abruptly, intermediate stages can be produced. Such a steel has an adaptable range of properties extending from strong/ductile/weakly ferromagnetic to high-strength/moderately ductile/ferromagnetic. The strength or ductility of the steel which it is possible to achieve can be increased further by what is known as the TWIP (Twinning Induced Plasticity) effect. The area of application of such materials is the production of lightweight steels which have high strength with low weight and good deformability.

[0005] The object of the present invention is to improve the applicability of the steel material described and to make possible the production of novel products from this material.

[0006] According to the invention, this object is achieved by virtue of the fact that, in a method of the type mentioned in the introduction, the product is manufactured locally differently by modification with regard to said phases with different properties.

[0007] This object is also achieved by virtue of the fact that a product of the type mentioned in the introduction has, over its surface and/or its thickness, at least two different areas, adapted by modification with regard to said phases, with different properties of the steel.

[0008] The essence of the present invention consists in locally varying the different properties of a product which can be adapted by virtue of the phase transition, so that the product is produced with portions with different properties by virtue of the phase adaptation.

[0009] This is possible by virtue of the mechanical stresses which lead to the phase transformation being exerted in only a locally limited manner. As the local exertion of stresses in a material is not always possible without problems, a preferred embodiment of the method according to the invention consists in that the product is firstly completely or partly transformed into the second phase and the product is then, on at least a part area, subjected to local thermal action in such a way that the steel in this part area is at least partly returned into the first phase. This method is therefore based on the knowledge that the phase transformation from the first phase into the second phase is reversible, and in particular by thermal action. When the steel material is heated to the austenite temperature (A_(l)), the transformation back into the first phase is successful, grain-refining generally taking place. It is therefore possible firstly to shape the product according to the invention, the transformation into the second phase taking place to a greater or lesser extent uniformly, and then to adapt the properties of the product by local heating operations. If appropriate, a further deformation can then be carried out. Carrying out a local heating operation is far less problematic than the exertion of locally limited stresses. In particular forming processes, sandblasting and ultrasound operations are possible mechanical actions for the transition into the second phase. The thermal action can take place by means of, for example, conventional heat sources, laser, microwaves, radiant heat and inductive heat generation.

[0010] Preferred steel materials are those which have a TRIP or TWIP effect. Use is therefore preferably made of a steel material with a composition of 10 to 30% manganese, 0 to 6% silicon and 0 to 8% aluminum. Additional austenite-stabilizing alloying elements, such as Cr and Ni, each up to 6%, are preferably included. These elements can, owing to their austenite-stabilizing property, partly replace Mn and at the same time have advantageous effects such as, for example, corrosion-inhibiting properties. Other usual steel-accompanying elements are possible.

[0011] In such a steel material, the first phase is formed solely or predominantly by metastable austenite, and the second phase solely or predominantly from bainite and/or martensite. In this connection, the first phase and the second phase also have different magnetic material properties. In particular, the first phase may be only weakly ferromagnetic, and the second phase may be much more strongly ferromagnetic.

[0012] The nature of the material makes possible a self-regulating local thermal action, which is carried out inductively. When the material of the product has been transformed fully into the second phase, the material is more strongly ferromagnetic and allows good feed-in of a magnetic induction field. If the induction field is strong enough, a transformation back into the first at most only weakly ferromagnetic phase occurs when the local heating of the product associated therewith takes place, as a result of which the feed-in of the induction field is reduced. By virtue of this, the effective heating of the product is reduced, and an equilibrium is brought about, in which some of the martensite or bainite is not transformed back into austenite.

[0013] A controlled transformation of bainite or martensite back into metastable austenite is also successful if another type of heat source is used, for example laser heating. In this case, it is advantageous additionally to generate a magnetic field on one side of the product and preferably to measure a magnetic leakage field with a magnetic leakage field sensor on the other side of the product. As long as the material is strongly ferromagnetic, it will not be possible to measure a leakage field through the product. If the transformation of the material from the second phase into the first phase has to a great extent been carried out, the feed-in of the induction field into the product is reduced, and a leakage field becomes measurable on the other side of the product. According to the desired degree of back-transformation, the further heating can consequently be continued, reduced or discontinued.

[0014] Instead of measuring a leakage field, a measuring field transmitted specifically for the purpose can be used, by means of which a permeability measurement of the material of the product is carried out.

[0015] The present invention makes it possible to produce very different products in an advantageous manner for very different applications. According to the invention, products with defined portions with particular properties such as, for example, magnetic properties, a particular ductility of the material for deformation purposes, and with alternating rigid and soft portions for bringing about a defined crash behavior, can be constructed.

[0016] Products according to the invention can be finished products, such as beams, motor car body parts etc., but also semi-finished products, such as steel strips, sheets and profiles.

[0017] In order to explain the invention further and to describe advantageous embodiments, illustrative embodiments are shown diagrammatically in the accompanying drawing, in which:

[0018]FIG. 1a shows a diagrammatic illustration of a steel strip with an inductor winding arranged in the strip center,

[0019]FIG. 1b shows a diagrammatic illustration of the field lines and currents which arise for a ferromagnetic strip,

[0020]FIG. 1c shows a diagrammatic illustration of the field lines and currents which arise in the case of a non-ferromagnetic strip,

[0021]FIG. 2a shows an illustration similar to FIG. 1b with a leakage field sensor arranged underneath the strip in the case of a ferromagnetic strip,

[0022]FIG. 2b shows the arrangement according to FIG. 2a in the case of a non-ferromagnetic strip,

[0023]FIG. 3a shows an arrangement for inductive local heating of a strip with a separate transmitting coil arranged on one side of the strip and a measuring coil arranged on the other side of the strip in the case of a ferromagnetic strip,

[0024]FIG. 3b shows the arrangement according to FIG. 3a in the case of a non-ferromagnetic strip,

[0025]FIG. 4a shows a deep-drawing tool with a sheet to be deep-drawn with a different property area,

[0026]FIG. 4b shows a diagrammatic illustration of the sheet after the deep-drawing operation,

[0027]FIG. 5 shows views of the stages of production of a profile by the formation of different areas in combination with stretching operations,

[0028]FIG. 6a shows a sheet with different property areas,

[0029]FIG. 6b shows the sheet from FIG. 6a after bending,

[0030]FIG. 7a shows a conventional tubular beam,

[0031]FIG. 7b shows a conventional tubular beam collapsed by mechanical loading,

[0032]FIG. 8a shows a tubular beam according to the invention with zones of different strength, and

[0033]FIG. 8b shows the tubular beam according to FIG. 8b collapsed by mechanical stress.

[0034]FIG. 1a shows a strip-shaped product 1, above which an induction coil 2 with high-frequency current flowing through it is arranged. Owing to the current flowing through turns 3 of the induction coil 2, a magnetic field 4 is induced, the magnetic field lines of which are illustrated in FIG. 1b. The strip-shaped product 1 has been transformed completely into the second, ferromagnetic phase, so that the magnetic field lines of the magnetic field 4 are concentrated in the product 1 and lead to strong heating with an induced current flow 5. FIG. 1b illustrates diagrammatically that the direction of the induced current 4 is opposite to the current flow in the turns 3 of the induction coil 2.

[0035] When the area of the strip-shaped product 1 is heated by the induction current 5 to such an extent that a return into the first, austenitic phase takes place, the strip-shaped product 1 thus heated becomes non-ferromagnetic in the heated area, in general austenitic-paramagnetic. This results in a shape of the magnetic field 4 as illustrated in FIG. 1c, by means of which only weak formation of induction currents 5 takes place.

[0036] The shape of the induction coil 2 can be adapted to the area to be heated of the strip-shaped product 1. Alternatively or additionally, the induction coil 2 can be moved over the product 1.

[0037] The adaptability of the martensite/bainite content can be improved further if the inductor output and/or the frequency of the high-frequency current is regulated.

[0038]FIG. 2a shows the arrangement according to FIG. 1b but with a sensor coil 6 arranged underneath the strip-shaped product 1 for measuring a leakage field.

[0039] As long as the product 1 is ferromagnetic, virtually no leakage field is present at the coil 6, as the magnetic field 4 is concentrated in the product 1. When the product 1 becomes paramagnetic (corresponding to FIG. 1c) as a result of the heating associated therewith, a leakage field according to FIG. 2b passes to the sensor coil 6, as a result of which a measuring signal characterizing the phase transition from the second phase to the first phase is generated.

[0040] The arrangement according to FIGS. 3a and 3 b shows a transmitting coil 7 which is independent of the induction coil. The field of the transmitting coil is measured by the measuring coil 6 arranged on the other side of the product 1, when the permeability of the product 1 decreases as a result of the transition into the paramagnetic first phase.

[0041]FIG. 4a and FIG. 4b show an advantageous arrangement of a strip-shaped product 1 for a deep-drawing process with a plunger 8 and a deep-drawing die 9.

[0042] In this connection, the product 1 is, according to the invention, divided into three areas 10, 11, a central area 10 acted on by the plunger 8 being relatively soft with high ductility, while the two edge areas 11 have higher strength and lower ductility. By means of the drawing tool 8, 9, a pot-shaped deformation of the product 1 is brought about according to FIG. 4b, where the central area 10 becomes a thin pot bottom, while the edge areas 11 form more thick-walled edge areas.

[0043] By virtue of the mechanical stress, the thin-walled pot bottom hardens in the central area 10, so that a strong product is produced, which, owing to the initial configuration according to FIG. 4a, makes possible a special wall thickness configuration according to FIG. 4b.

[0044]FIG. 5 shows a special wall thickness configuration which is produced when stretching operations are carried out. FIG. 5 shows in one view a product 1 which is likewise divided into three areas 10, 11, with a central soft area 10 and two stronger edge areas 11. Stretching tongs 12, which stretch the product 1, take hold of the stronger edge areas 11. By means of the stretching operation, a stretched thin-walled central area 10′ is produced from the soft area 10, while the side areas 11, as less ductile areas, remain almost unchanged.

[0045] In the next illustration in FIG. 5 in the transverse direction, the profile is divided by local heat treatment into three strong areas 11 and one soft area 10, the soft area 10 completely surrounding a central strong area 11. By means of transverse stretching tongs 13, transverse stretching is carried out, which produces, above and below the central strong area 11, a transverse profile with a central stretched area 10′ and strong edge areas 11, the central area 10′ being hardened by the stretching operation (transverse profile A).

[0046] At the level of the central strong area 11, on the other hand, two stretched portions 10′ are formed from the soft area 10 between the strong end portions 11 and the strong central area 11. A transverse profile as is illustrated as transverse profile B in FIG. 5 is consequently produced.

[0047] Products 1 with different wall thicknesses can consequently be specifically manufactured by stretching and heat treatment.

[0048]FIG. 6 illustrates the shaping of a product 1 with a central soft area 10 and two strong edge areas 11. A U-shaped product 1 is produced by bending, which product can easily be deformed into the U-shape in the soft area 10 and in this way forms a deformed area 10′ which is hardened by the deformation.

[0049] The method according to the invention can be utilized to form products 1 with advantageous deformation behavior. FIG. 7a shows a conventional tubular beam 14 of circular cross section A. If this tubular beam 14 collapses owing to excessive mechanical pressure, its cross section A′ in the collapsed area is deformed considerably, the tubular beam 14 being capable of absorbing only a limited amount of mechanical energy by virtue of its deformation behavior.

[0050] A tubular beam 15 according to the invention, on the other hand, consists of soft areas 10 and strong areas 11 which are arranged in an annular manner alternately in the longitudinal direction and have circular cross sections B, C.

[0051] In the event of this tubular beam 15 collapsing, the strong areas 11 are barely deformed, which is illustrated by the still virtually circular cross section B′ in FIG. 8b. On the other hand, the soft areas 10′ are deformed greatly and form star-like cross sections C′ which have a high deformation content and thus bring about high energy absorption by virtue of the deformation. The product 15 according to the invention is therefore particularly suitable for axial energy absorption in a crash situation.

[0052] A similar effect to that achieved by means of the annular zones illustrated can be achieved using helically extending zones 10, 11.

[0053] The arrangement of zones of high strength in the longitudinal direction in a tube results in high resistance to axial deformation with a soft, that is energy-dissipating, torsional behavior.

[0054] The products 1, 15 according to the invention can therefore be “tailored” to different applications and open up completely new advantageous perspectives with simple manufacturing methods. 

1. A method of manufacturing a product (1, 15) from steel, in which a steel with a metastable first phase in which the steel has first properties is manufactured and, by mechanical influences, is transformed into a second, thermodynamically more stable phase with second properties which are at least partly different from the first properties, wherein the building product (1, 15) is manufactured locally differently by modification with regard to said phases with different properties.
 2. The method as claimed in claim 1, wherein the product (1, 15) is firstly completely or partly transformed into the second phase and wherein the product (1, 15) is then, on at least a part area, subjected to local thermal action in such a way that the steel in this part area is at least partly returned into the first phase.
 3. The method as claimed in claim 1 or 2, wherein the metastable first phase is an austenite and the second phase is bainite or martensite.
 4. The method as claimed in claim 2 or 3, wherein the local thermal action is carried out inductively.
 5. The method as claimed in claim 2 or 3, wherein the local thermal action is carried out by means of a heat source directed locally at the product (1, 15) in a controlled manner.
 6. The method as claimed in claim 5, wherein additionally a magnetic field is generated and directed at the product (1, 15).
 7. The method as claimed in claim 4 or 6, wherein the thermal action is controlled with the aid of a sensor coil (6).
 8. A steel product which, by virtue of its composition and its manufacture, has a metastable first phase in which the steel has first properties and which, by mechanical influences, can be transformed into a thermodynamically more stable, second phase with second properties which are at least partly different from the first properties, wherein, over its surface and/or its thickness, it has at least two different areas (10, 11), adapted by modification with regard to said phases, with different properties of the steel.
 9. The steel product as claimed in claim 8, wherein the first phase is formed by metastable austenite and the second phase by bainite and/or martensite.
 10. The steel product as claimed in claim 9, wherein the first and/or second phase is formed from a mixture of metastable austenite and bainite and/or martensite.
 11. The steel product as claimed in one of claims 8 to 10, wherein the first phase and the second phase have different material properties.
 12. The steel product as claimed in claim 11, wherein the first phase is not or is only weakly ferromagnetic and the second phase is much more strongly ferromagnetic.
 13. The steel product as claimed in one of claims 8 to 12, in the form of a strip-shaped product (1) with areas (10, 11) which are different with regard to hardness extending over a dimension of the strip.
 14. The steel product as claimed in one of claims 8 to 12, in the form of an elongate beam with areas (10, 11) of high and low strength formed in the longitudinal direction.
 15. The steel product as claimed in one of claims 8 to 12, in the form of an elongate beam with areas (10, 11) of different strength adjacent to one another in planes located at right angles to the longitudinal direction. 